1. Field of the Invention
The present invention relates to a linear guide apparatus which is used in a machine tool and an industrial machine.
2. Description of the Related Art
To guide a table of a machine tool or an industrial machine which can be moved at a relatively high speed by a ball screw or a linear motor, there is used such a linear guide apparatus 51 as shown in FIG. 8.
The linear guide apparatus 51 shown in FIG. 8, which is generally called a linear guide, comprises guide rails 53 serving as guide members respectively extending in one direction, and sliders 54 movably assembled to their associated guide rails 53. In the outer surface of each guide rail 53, there is formed a rolling body rolling groove 58 which serves as a guide surface; in each slider 54, there is formed a load rolling body rolling groove serving as a load guide surface in such a manner that it faces the rolling body rolling groove 58 of the guide rail 53; between the rolling body rolling groove 58 and load rolling body rolling groove, there are interposed a large number of balls serving as rolling bodies; and, the slider 54 can be moved along the guide rail through these rolling bodies.
As the guide rails 53, there are used a pair of guide rails 53 which are arranged parallel to each other, and they are respectively mounted on a bed 59 which is generally placed on a floor. In the case of the sliders 54, a pair of sliders 54 are assembled to each guide rail 53 in such a manner that they are spaced from each other and can be moved along the longitudinal direction of the associated guide rail 53. The sliders 54 are respectively mounted on a table 52.
A drive mechanism 55, which is used to drive the table 52, includes a screw shaft 60 and a ball nut 61. The screw shaft 60 includes a screw groove formed on the outer surface thereof and is mounted on the bed 59 along the guide rails 53 in such a manner that it can be rotated about its own axis. The ball nut 61 is threadedly engaged with the screw shaft 60 through a large number of balls serving as the rolling bodies and is mounted on the table 52. The drive mechanism 55 is structured such that it can rotate the screw shaft 60 to thereby move the table 52 along the guide rails 53 through the ball nut 61.
Also, the linear guide apparatus 51 includes a friction apply unit (not shown) which is used to stop the slider 54 at a given position of the guide rail 53. The friction apply unit includes a brake member disposed so as to be opposed to the upper surface or side surface of the guide rail 53 or to the rolling body rolling groove 58, and a drive device which is capable of pressing the brake member toward the guide rail 53. The brake member is structured such that, in case where it is pressed toward the guide rail 53 by the drive means and is thereby contacted with the guide rail 53, it can generate a frictional force.
The friction apply unit pushes the brake member using the drive device to apply a frictional force to the slider 54 and table 52 to thereby set them at their given positions of the guide rail 53. By driving the friction apply unit, the damping characteristic of the vibrations of the slider 54 and table 52 can be enhanced and thus the rigidity of the slider 54 and table 52 can be enhanced.
However, in the thus structured linear guide apparatus 51, when the brake member is pressed against the upper surface of the guide rail 53 to thereby generate a frictional force, the slider 54 and table 52 are pushed up as the brake member is pressed against the guide rail 53, which degrades the positioning accuracy of the slider 54 and table 52.
In view of this problem, in Japanese Patent Unexamined Publication No. Hei. 7-54845 (JP-A-7-54845), there is disclosed a linear guide apparatus in which the brake member is fitted into the rolling body rolling groove 58 of the side surface of the guide rail 53 and the brake member is pressed by an oil pressure cylinder. In this case, since the brake member is fitted into the rolling body rolling groove 58, there is no possibility that the slider 54 and table 52 are pushed up when braking. However, because the brake member is pressed against the rolling body rolling groove 58 to thereby a frictional force, there is a tendency that the rolling body rolling groove 58 is worn to such a degree that cannot be ignored, which results in the degraded positioning accuracy of the slider 54 and table 52.
Also, in the above-cited publication JP-A-7-54845, when removing the braked condition, the oil pressure of the oil pressure cylinder is removed to thereby produce a non-load condition and thus the brake member and the piston of the oil pressure cylinder are retreated due to the elastic force of a coiled spring.
Here, in such structure where the brake member and the piston of the oil pressure cylinder are retreated due to the elastic force of a coiled spring, in case where the response characteristic (speed) of removal of the braked condition is enhanced to thereby control the frictional force with high accuracy, the elastic force of the coiled spring must be set fairly large.
However, in case where the elastic force of the coiled spring is set large, there increases the load that is produced when the brake member is pressed by the oil cylinder against the elastic force of the coiled spring, which degrades the response characteristic of the brake member when it is pressed.
Further, as in the above-cited publication JP-A-7-54845, in the case of a structure in which a brake member (brake shoe) is slidably fitted into a sliding groove formed in a damping block, there exists a clearance between the sliding groove and brake member and this clearance causes the slider 54 to play in the advancing direction thereof. This invites the delayed generation of the frictional force, which makes it difficult to control the frictional force with high accuracy.